The well-known uncertainties which still exist in the mechanism 1) of the transformation, by the testis, of the C-21 steroids to the active androgens, 2) of the transformation by the adrenal cortex, of the C-18 desoxysteroids to aldosterone, and 3) of the transformation, by the ovaries, of the C-19 steroids to the active estrogens, pertain also to the transformation of a) cholesterol to pregnenolone in endocrine glands and b) to bile acids in the liver. Both the biosynthesis of pregnenolone, which is an intermediate for the syntheses of corticoids and sex hormones, and also the transformation of cholesterol to the bile acids in the liver which is known to proceed through distinctive steps, will be investigated qualitatively and quantitatively. We will investigate proposed kinetic data, particularly in view of a suggested compartmental division of enzyme and substrate which will be pertinent for the study of the action of trophic hormones. The quantitative aspects of compartmentalization will be studied with molecules marked with deuterium and tritium, while kinetic data will be obtained with sterols and bile acids marked with tritium and carbon-14. Pathways for the conversion of cholesterol to the bile acids during normal and abnormal growth will make it possible to define alterations in this conversion, representative of essential changes in the neoplastic state (Novikoff hepatoma). Hepatic cell membranes from normal liver will be incubated with cholesterol catabolites and the resulting interference to bind insulin will be studied. This will be compared to hepatic samples from maturity-onset diabetics, in search of an abnormal binding of insulin.